This invention relates to a tool component and the use thereof, specifically to a tool component with enhanced wear resistance.
Boron nitride exists typically in three crystalline forms, namely cubic boron nitride (cBN), hexagonal boron nitride (hBN) and wurtzitic cubic boron nitride (wBN). Cubic boron nitride is a hard zinc blend form of boron nitride that has a similar structure to that of diamond. In the cBN structure, the bonds that form between the atoms are strong, mainly covalent tetrahedral bonds. Methods for preparing cBN are well known in the art. One such method is subjecting hBN to very high pressures and temperatures, in the presence of a specific catalytic additive material, which may include the alkali metals, alkaline earth metals, lead, tin and nitrides of these metals. When the temperature and pressure are decreased, cBN may be recovered.
cBN has wide commercial application in machining tools and the like. It may be used as an abrasive particle in grinding wheels, cutting tools and the like or bonded to a tool body to form a tool insert using conventional electroplating techniques.
cBN may also be used in bonded form as a cBN compact, also known as PCBN. cBN compacts tend to have good abrasive wear, are thermally stable, have a high thermal conductivity, good impact resistance and have a low coefficient of friction when in contact with a ferrous workpiece.
Diamond is the only known material that is harder than cBN. However, as diamond tends to react with certain materials such as iron, it cannot be used when working with iron containing metals and therefore use of cBN in these instances is preferable.
cBN compacts comprise sintered polycrystalline masses of cBN particles. When the cBN content exceeds 80 percent by volume of the compact, there is a considerable amount of direct cBN-to-cBN contact and bonding. When the cBN content is lower, e.g. in the region of 40 to 60 percent by volume of the compact, then the extent of direct cBN-to-cBN contact and bonding is less.
cBN compacts will generally also contain a binder phase which may be a cBN catalyst or may contain such a catalyst. Examples of suitable binder phases are aluminum, alkali metals, cobalt, nickel, and tungsten.
When the cBN content of the compact is less than 75 percent by volume there is generally present another hard phase, a third phase, which may be ceramic in nature. Examples of suitable ceramic hard phases are nitrides, borides and carbonitrides of a Group IVA or VB transition metal, aluminum oxide, and carbides such as tungsten carbide and mixtures thereof.
cBN compacts may be bonded directly to a tool body in the formation of a tool insert or tool. However, for many applications it is preferable that the compact is bonded to a substrate/support material, forming a supported compact structure, and then the supported compact structure is bonded to a tool body. The substrate/support material is typically a cemented metal carbide that is bonded together with a binder such as cobalt, nickel, iron or a mixture or alloy thereof. The metal carbide particles may comprise tungsten, titanium or tantalum carbide particles or a mixture thereof.
A known method for manufacturing the polycrystalline cBN compacts and supported compact structures involves subjecting an unsintered mass of cBN particles to high temperature and high pressure conditions, i.e. conditions at which the cBN is crystallographically stable, for a suitable time period. A binder phase may be used to enhance the bonding of the particles. Typical conditions of high pressure and temperature (HPHT) which are used are pressures of the order of 2 GPa or higher and temperatures in the region of 1100° C. or higher. The time period for maintaining these conditions is typically about 3 to 120 minutes.
The sintered cBN compact, with or without substrate, is often cut into the desired size and/or shape of the particular cutting or drilling tool to be used and then mounted onto a tool body utilising brazing techniques.
The cBN abrasive compacts, although performing acceptably, require continuing improvement in their properties to meet the need for better tool lifetimes and lower costs, and research and development are ongoing to provide such improvements in the marketplace.
cBN abrasive compacts are used in high-speed machining of hard ferrous materials such as die steels, alloy steels and hard-facing materials. The main advantage of high-speed hard turning is the elimination of expensive and time consuming grinding operation to finish the part. cBN abrasive compacts are the most suitable cutting tools for high-speed, hard-turning operations.
In high speed machining of hardened steels increased hardness of the work piece results in higher than usual cutting forces, stresses and temperatures at the cutting zone. In particular wear behaviour of a cBN cutting tool is very sensitive to temperatures developed at the chip-tool and workpiece tool interfaces. Elevated temperatures at the chip-tool interface causes accelerated wear mainly by chemical wear leading to a deep crater formation on the rake face of the tool. This results in formation of a sharpened cutting edge which is prone to chipping or fracture. In most cases the deep crater breaks the cutting edge with continuous wear, leading to a catastrophic failure of the cutting tool by edge chipping.
This is illustrated by the attached FIG. 1. Referring to FIG. 1, a tool component comprises a layer 10 of polycrystalline cBN material which has a rake (working) surface 12 and a flank surface 14. The cutting edge of the tool component, prior to use, is the edge 16. During use, a deep crater 18 forms and the flank surface 14 wears to form surface 20. Sharpened cutting edge 22 results.
In industry there is a drive towards ever increasing cutting speeds to improve throughput and productivity and hence severe crater wear formation is one of the biggest factors affecting the overall performance of cBN abrasive compact cutting tool and machining economics. Therefore, it is expected that any reduction in crater wear will not only result in longer tool life but also it will give the tool opportunity to be used at a higher cutting speed.
EP 102843 describes the use of a thin, wear-resistant refractory layer bonded to a PCBN tool insert where the cBN content is in excess of 70 vol %. The refractory layer is preferably titanium nitride or carbide, or a mixture thereof, and is typically less than 20 microns thick. It is applied after the PCBN tool is sintered and processed using a method such as CVD. High cBN PCBN is used in applications like turning or milling, which require a high degree of abrasion resistance. These applications are carried out at lower speeds (i.e. the tool does not get as hot) and the cBN is not compromised by exposure to chemically aggressive systems at high temperatures. By contrast, low cBN tools are used in high tool speed applications where failure due to crater wear is a major problem. High cBN content PCBN does not perform sufficiently well in these high speed, chemically demanding applications, because of a lack of chemical resistance. Whilst high cBN content PCBN may experience some degree of crater wear in their standard applications, it is never the dominant failure mode, as is the case with the low cBN materials.